Sheila Gold, supposedly Britain’s most accurate psychic, wants to be taken seriously by her new clients. “This is not theatre,” she warns them, as she lights seven candles for a seance.

This is an insider joke. Theatre is exactly what it is.

It is the kind of theatre in which the lights dim, then dim some more, a bell tinkles of its own accord and drowns out the ticking of a clock, while the audience sinks into a special kind of silence, still and focused. If it were not theatre, there would be no illusion.

The Psychic marks the return of Jeremy Dyson and Andy Nyman after the spooky stage and screen success of Ghost Stories. Now the writer-directors are unnerving audiences again with grinding sound effects and sudden lighting bursts, toying with us to believe and yet not believe in voices from beyond the grave.

One minute, they expose Sheila as a fraud, trying to claw back a £500,000 legal bill, and the next, they seduce us into believing there could be something in this spiritualism lark after all. In the lead role, Eileen Walsh does an excellent job switching from glitzy entertainer in sparkling pink jacket and matching heels to hard-bitten operator, building on the received wisdom of 10 generations of fortune tellers.

Where Ghost Stories was an all-male affair, this one is a female-centred tale in which 18-year-old Tara (Megan Placito) tries to inherit Sheila’s fairground wisdom, while matriarch Rosa (Frances Barber) does all she can to undermine the daughter she schooled. The script is littered with the language of showmen, from the jossers whose fortunes they read to the oojas who mentor them, adding to the impression of ancient knowledge being passed down.

Dyson and Nyman request that the audience keep the plot twists secret. This is reasonable in the first half where nothing is what it seems. But in the second, the ideas dry up. With little to surprise us beyond the odd jarring sound effect, the play drifts towards Victorian melodrama: too much expository dialogue, too little tension and an ending that is uncharacteristically predictable.

• At York Theatre Royal until 23 May